Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving surface. The ink can be stored in reservoirs that are located within cartridges installed in the printer. Such ink can be aqueous ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the imaging member. In these solid ink printers, the solid ink can be in the form of pellets, ink sticks, granules, pastilles, or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device, which melts the solid ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. Other inkjet printers use gel ink. Gel ink is provided in gelatinous form, which is heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead. Once the melted solid ink or the gel ink is ejected onto the image receiving surface, the ink returns to a solid, but malleable form, in the case of melted solid ink, and to gelatinous state, in the case of gel ink.
A typical inkjet printer uses one or more printheads with each printhead containing an array of individual nozzles through which drops of ink are ejected by inkjets across an open gap to an image receiving surface to form an ink image. The image receiving surface can be a continuous web of recording media, a series of media sheets, or the image receiving surface can be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an aperture, usually called a nozzle, in a faceplate of the printhead. The actuators expel an ink drop in response to an electrical signal, sometimes called a firing signal. The magnitude, or voltage level, of the firing signals affects the amount of ink ejected in an ink drop. The firing signal is generated by a printhead controller with reference to image data. A print engine in an inkjet printer processes the image data to identify which inkjets in the printheads of the printer are operated to eject a pattern of ink drops at particular locations on the image receiving surface to form an ink image corresponding to the image data. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to electronic image data.
In order for the printed images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads are registered with reference to the imaging surface and with the other printheads in the printer. Registration of printheads refers to a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the relative positions of the printheads with reference to the imaging surface and with reference to the other printheads in the printer. Operating the printheads in a printer to eject ink in correspondence with image data presumes that the printheads are level with one another across a width of the image receiving surface and that all of the inkjets in the printhead are operational. The presumptions regarding the positions of the printheads, however, cannot be assumed, but must be verified. Additionally, if the conditions for proper operation of the printheads cannot be verified, the analysis of the printed image should generate data that can be used either to adjust the printheads so they better conform to the presumed conditions for printing or to compensate for the deviations of the printheads from the presumed conditions.
During operation, one or more inkjets in the printheads may become inoperable. An inoperable inkjet includes any inkjet that fails to eject ink drops on demand, ejects ink drops only intermittently, or ejects ink drops onto an incorrect location on the image receiving surface. Inoperable inkjets in a print zone can produce defects and artifacts in printed images. Some printers detect inoperable inkjets during a print job and compensate for the inoperable inkjets until the printheads containing the inoperable inkjets are cleaned or serviced. Scanned image data from printed patterns that are formed on the image receiving surface are used for both registration of the printheads and for identification of inoperable inkjets.
Analysis of printed images is performed with reference to two directions. “Process direction” refers to the direction in which the image receiving surface is moving as the imaging surface passes the printhead to receive the ejected ink and “cross-process direction” refers to an axis that extends across the width of the image receiving surface, which is perpendicular to the process direction. In order to analyze a printed image, a test pattern needs to be generated in a manner that enables determinations to be made as to whether the inkjets operated to eject ink did, in fact, eject ink and whether the ejected ink landed where the ink would have landed if the printhead was positioned correctly with reference to the image receiving surface and the other printheads in the printer. In some printers, an optical scanner is integrated into the printer and positioned at a location in the printer that enables the scanner to generate image data corresponding to the ink image while the image is on media within the printer or while the ink image is on the rotating image receiving surface in the printer.
These integrated scanners typically include one or more illumination sources and a plurality of optical detectors that receive radiation from the illumination source that has been reflected from the image receiving surface. The radiation from the illumination source is usually visible light, but the radiation can be at or beyond either end of the visible light spectrum. If light is reflected by a white surface, the reflected light has the same spectrum as the illuminating light. In some systems, ink on the imaging surface can absorb a portion of the incident light, which causes the reflected light to have a different spectrum. In addition, some inks may emit radiation in a different wavelength than the illuminating radiation, such as when an ink fluoresces in response to a stimulating radiation. Each optical sensor generates an electrical signal that corresponds to the reflected light received by the detector. The electrical signals from the optical detectors are converted to digital signals by analog to digital converters and provided as digital image data to an image processor.
In many high-volume printers, the image receiving surface moves past the printheads and the optical scanner at high speed in the process direction. For example, some continuous media printers include a media web that moves past the printheads and the optical scanner at a rate of several hundred feet per minute. The optical scanner is only activated for brief periods to capture scanned images of the printed test patterns on the media web while being deactivated when printed images on the media web pass the optical scanner. If the scanned image data include portions of printed images, the registration and inoperable inkjet detection processes may become less effective since the scanned images can be confused with the printed test patterns. Additionally, the printhead registration and inoperable inkjet detection processes are less effective if the optical scanner only captures a portion of the printed test pattern. During operation, small changes in the media web including slip and web shrinkage introduce small errors in synchronization between the locations of the test patterns on the media web and the optical sensor. As the errors accumulate, the optical scanner may capture portions of the media web that include the printed image or may fail to capture the entire printed test pattern. Consequently, improvements to the synchronization of operation for the optical scanner to enable accurate generation of scanned image data for printed test patterns would be beneficial.